In recent years, in the welding industry, the need for improving productivity of welding work and improving product quality has been increased. In particular, automobile and motorcycle industries, the need for improving production efficiency by producing a larger number of materials to be welded (hereinafter, referred to as a “workpiece”) for a short time than before, reducing production cost per workpiece, and reducing occurrence of defective pieces such as those with lack of weld beads or those with holes so as to improve the yield has been increased. In order to meet such needs, it is extremely important to stabilize the arc length during welding.
However, due to displacement of workpieces or inaccuracy of workpieces, a gap occurs between the workpieces, and thus the length of wire extension is changed. As a result, the arc length is changed. Furthermore, the arc length is changed also due to motion of the hands of a welding operator. Since the arc length is changed due to such external influences, the welding quality cannot be improved.
In consumable electrode type arc welding, a plurality of output characteristics of a welding power-supply device to respond to the arc length change generated by various external influences including the above-mentioned examples have been conventionally known.
One of such characteristics is, as shown by a solid line in FIG. 5, a right downward sloping characteristic in which as an output current is increased, an output voltage is reduced. According to this characteristic, the self-regulation of the arc length works, so that the arc length becomes constant. The reducing gradient is generally about 1 V to 3 V/100 A. However, it may be 0 V/100 A or about 10 V/100 A depending on the welding method, welding wire, and the like.
In FIG. 5, for example, when the arc length is shortened by an external influence from state (P1) in which welding is carried out under the welding conditions of output current A1 and output voltage V1, the output current is increased from A1 to A2 and the output voltage is reduced from V1 to V2, and the state is shifted from P1 to P2 on the output characteristic. However, when the output current is increased, a melting amount of the wire is increased and the arc length is gradually increased to return to P1. This is called a self-regulation.
In detail, when a wire is fed at a constant speed by using a welding power-supply device having a constant voltage characteristic, and metal inert gas (MIG) welding or metal active gas (MAG) welding is carried out, a phenomenon in which the arc length automatically becomes a predetermined value occurs. This is called a self-regulation. However, in response to changes of the arc length due to various external influences, the self-regulation takes a long time to return the arc length to the original arc length.
Furthermore, in pulse arc welding, according to a conventional output control method, when a change of the arc length due to an external influence is large, at least one of a base current or a peak current is increased or reduced with a difference between a welding output voltage and a set welding voltage (see, for example, patent document 1). In this control method, when an arc becomes unstable, that is, when the welding output voltage is largely changed due to an external influence, the change of pulse frequency can be suppressed by increasing or reducing a base current or a peak current for correcting large change amount. Thus, the arc length can be stabilized and occurrence of irregular short-circuit can be suppressed so as to suppress spatters. However, this output control method also takes a long time to return the arc length to the original arc length.    [Patent Document] Japanese Patent Application Unexamined Publication No. 2004-237342